Field of Endeavor
This invention relates to shape memory polymers and specifically to shape memory polymers and polymer foams having enhanced characteristics of degradability, control over cell structure, and density.
State of Technology
Shape memory polymers (SMPs) are materials which can remember two or more shapes, and can be actuated to go from one shape to another via a stimuli involving heat or light etc. Thermally responsive SMPs that use heat energy for their actuation can be deformed from their primary shape to a secondary shape above their actuation temperature. This secondary shape can then be “fixed” by cooling the deformed shape to below the material's actuation temperature. When they are heated to above their actuation temperature on demand, they recover their “remembered” primary shape. Polyurethane based SMP foams were initially proposed by Hayashi et. al—Japanese patent 5049591 (1991). Other related patent applications were also filed in this field by Applicants, U.S. patent application Ser. No. 10/801,355 (2004) and U.S. patent Patent Application number 20060036045 (2005). These shape memory materials are useful in diverse applications like shape adaptive sportswear (helmets, suits), housing (thermal sealing of doors and windows) and robotics (conformal grip design). Also these materials are being investigated for use in automobile and aerospace industries for self healing automobile bodies and morphing aircraft wings. In addition, shape memory foam based biomedical devices for minimally invasive surgeries are being developed, see El Feninat, F., Laroche, G., Fiset, M. & Mantovani, D. Shape memory materials for biomedical applications. Advanced engineering materials 4, 91-104 (2002); Sokolowski, W., Metcalfe, A., Hayashi, S., Yahia, L. H. & Raymond, J. Medical applications of shape memory polymers. Biomedical Materials 2, S23 (2007) and Small, W. et al. Shape memory polymer stent with expandable foam: a new concept for endovascular embolization of fusiform aneurysms. Biomedical Engineering, IEEE Transactions on 54, 1157-1160 (2007).
Introducing degradability is a key requirement for the development of a SMP either as a biomedical device to avoid long term presence of a foreign material in the human body, or for the ecological concerns of using polymeric materials. Lendlein, A. & Langer, R. Biodegradable, elastic shape-memory polymers for potential biomedical applications. Science 296, 1673 (2002) Several degradable SMPs have been reported with polycaprolactone diol, lactide and glycolide moieties. Caprolactone has been used copiously in the biodegradable applications and its degradation products have been shown to be benign in the earlier biocompatibility studies; Rickert, D., Lendlein, A., Kelch, S., Franke, R. & Moses, M. Cell proliferation and cellular activity of primary cell cultures of the oral cavity after cell seeding on the surface of a degradable, thermoplastic block copolymer. Biomedizinische Technik. Biomedical engineering 50, 92 (2005).
However, most of these materials reported earlier are either linear polymers, have relatively low covalent crosslink density, or are based on physical crosslinks, which can limit the mechanical and/or shape memory behavior of the material.
In contrast to above, a network structure consisting of high density of covalent crosslinks, is preferable for good mechanical properties and improved shape memory behavior (high recovery force, high shape recovery), particularly for very low density foams. This is so because, firstly, the modulus of a foamed material declines rapidly as its density is reduced: Eporous=Eneat[Qporous/Qneat] for open cell foams, where Eporous and Eneat are the Young's moduli and Qporous and Qneat are the densities, of porous and neat/unfoamed materials respectively. Hence for very low density materials to have good mechanical properties, they should be based on neat/unfoamed materials with significantly high modulus i.e. high density of crosslinks (from E˜3ncRT where E is the Young modulus of material, nc the number of active network chain segments per unit volume, R is the ideal gas constant and T is the Temperature). Secondly, it is important to have a covalently crosslinked structure, rather than a physically crosslinked structure for improved shape memory behavior to be retained over extended periods of time, i.e. to avoid secondary-shape forming phenomenon as is noticed in some physically crosslinked materials; Tobushi, H., Matsui, R., Hayashi, S. & Shimada, D. The influence of shape-holding conditions on shape recovery of polyurethane-shape memory polymer foams. Smart materials and structures 13, 881 (2004). Since physical crosslinks are labile, entropically driven polymer chains in a physically crosslinked material can move in and out of their crosslink sites to attain a more preferable, lower energy equilibrium conformation. This causes the material to lose the memory of its primary shape, and thus, lose its ability to actuate under a stimulus (as noticed in the secondary-shape forming phenomenon). In contrast, covalent/chemical crosslinks do not allow such rearrangement of entropically driven polymer chains, and thus ensure improved shape memory behavior even after extended periods of storage in the secondary shape. Hence a high crosslink density in the network structure, and use of chemical/covalent crosslinks for achieving the same, form the basis of the materials of this invention.
The method of synthesis of such a highly covalently crosslinked degradable polymer network based on polyurethane chemistry is an embodiment of this invention. Degradability is shown using multifunctional Polycaprolactone based hydroxyl monomers, which has not been proposed before in blown foams. Also other variations of monomers for developing a highly crosslinked network structure are disclosed as are methods of controlling material's actuation temperature and rate of degradation.
Controlling the cell structure of foams is another key requirement in generation of commercial grade SMP foams, and we propose manipulating viscosity of the foaming solution for the same. The effect of viscosity on foam cell structure has been studied in detail for foam emulsions; Kim, Y. H., Koczo, K. & Wasan, D. T. Dynamic Film and Interfacial Tensions in Emulsion and Foam Systems. Journal of colloid and interface science 187, 29-44 (1997) and Shah, D., Djabbarah, N. & Wasan, D. A correlation of foam stability with surface shear viscosity and area per molecule in mixed surfactant systems. Colloid & Polymer Science 256, 1002-1008 (1978)19-20 and in HIPE foam synthesis procedure; Busby, W., Cameron, N. R. & Jahoda, C. A. B. Emulsion-Derived Foams (PolyHIPEs) Containing Poly (-caprolactone) as Matrixes for Tissue Engineering. Biomacromolecules 2, 154-164 (2001) and Christenson, E. M., Soofi, W., Holm, J. L., Cameron, N. R. & Mikos, A. G. Biodegradable Fumarate-Based PolyHIPEs as Tissue Engineering Scaffolds. Biomacromolecules 8, 3806-3814, doi:10.1021/bm7007235 (2007). For blown foams, the indirect effect of change in viscosity on cell structure, via changing the functionality of polyols or chemistry of foams24 has been reported; Tabor, R., Lepovitz, J., Potts, W., Latham, D. & Latham, L. The effect of polyol functionality on water blown rigid foams. Journal of Cellular Plastics 33, 372 (1997).
However, using this technique to control the cell structure of very low density blown foams, keeping the net chemical composition the same has not been previously reported. Currently in literature, low density foams have been reported mostly down to the lower limit of 0.02-0.03 g/cc; see Thirumal, M., Khastgir, D., Singha, N. K., Manjunath, B. & Naik, Y. Effect of foam density on the properties of water blown rigid polyurethane foam. Journal of Applied Polymer Science 108, 1810-1817 (2008) and Simpson, S. S. & Sato, et al, U.S. Pat. No. 7,338,983 (2008).
A recent patent by Burdeniuc reported making foams down to 0.006 g/cc, but a staggering 75 wt % water was used in synthesis; Burdeniuc, J. J. & Andrew, G. D. Catalyst composition for water blown, low density, rigid polyurethane foam, US patent application 20100152312 (2008). Since use of high amounts of water as chemical blowing agent will interfere with the intended covalently crosslinked network structure of the material, this is not a preferred route. Another patent reported densities down to 0.016 g/cc by varying the polyol type and amount; Haider, K. W. et al. Polyol compositions useful for preparing dimensionally stable, low density water-blown rigid foams and the processes related thereto, U.S. Pat. No. 7,300,961 (2007). However, keeping the same chemical composition, large variation in densities with lower limit ˜0.005 g/cc has not been reported before.